4x4 wont engage

I have a 94 chevy 1500 with posi-lok 4x4. I can not get my 4-wheel drive to lock in. The transfer case will engage but the front tires will not move. I know nothing about the posi-lok system. I have visted their website and looked at the troubleshooting guide, it was of little use to me. I have checked the cable and it is fine, any ideas what is next?